Certain microbes, for example gram negative bacteria and algae, exude viscoelastic extracellular polymeric substances called exopolysaccharides (EPS) and these exopolysaccharides and their complexes with proteins (exolipoprotienpolysaccharide complexes or EPS/protein complexes) act to protect the microbial cell from external physical and chemical attacks. (Unless otherwise specified, the abbreviation EPS is used herein to refer to EPS materials and EPS/protein complexes collectively.) The EPS also are critical in cellular cohesion responsible for the formation of films, and EPS are involved in adhesion of the microbial cells to surfaces.
EPS materials and EPS/protein complexes are extremely viscoelastic, a feature that can cause unexpected yet severe problems in aqueous environments. For example, the viscoelasticity of less than one pound of brown algae on the total hydroplane surfaces of many naval destroyers is sufficient to reduce top speed from 22 knots to 18 knots and cause fuel consumption to increase by as much as 20%. The presence of these materials in sea water may cause reverse osmosis membranes to lose 20% of their flux within one hour of engagement due to the formation of a monomolecular layer of EPS, which can result in a four-fold increase in the polar component of the surface energy of the membrane.
Also by way of example, dramatic differences in surface energies, contact angles with the polar component water, and surface polarities of clean versus microbial-fouled reverse osmosis membranes have been observed. For example, a comparison of surface energy of clean versus fouled sea water reverse osmosis (SWRO) membranes reveals the following dramatic differences shown in the following table.
TABLE 1Comparison of Surface Energy of Clean and FouledSea Water Reverse Osmosis (SWRO) MembranesContact Angle (deg) withSurfaceSurfacepolar component (water)EnergyPolarityClean SWRO77.9°45.074.93%Fouled SWRO59.3°54.8715.94%
Conventionally, materials such as copper, silver, organic anti-microbial compounds, and other compositions and materials have been employed to prevent microbial growth. While the exact mechanisms by which these materials operate often are not understood, some compositions such as organic bactericides function primarily by interfering with cell wall formation. The interference with cellular metabolism and respiration is implicated in other materials, such as the metals. In effect, all of these conventional materials act in some manner as poisons, which also may impart adverse effects on human health and ecology. Therefore, coatings treated with these materials and their direct introduction into water is generally undesirable due to their negative effects on flora, fauna, and humans.
In order for microbes to reproduce and anchor to a surface, they must first exude an EPS “nest” which causes divided cells to cohere to each other, and provides the necessary conditions for them to anchor to a substrate. This EPS nest is critical to cell survival, and without such a structure, cells generally will wither and die. However, conventional antimicrobial treatments that produce dead cells also can be problematic because of the adverse effects of cell membrane decomposition products, for example the components of the cell walls of certain dead bacteria. These membrane decomposition products are known as endotoxins, and endotoxins can cause human diseases through inhalation of aerosols containing these materials, through ingestion, and through skin contact. Moreover, endotoxins may be one of the sources of EPS materials responsible for fouling filtration surfaces such as in membrane filtration devices and in anti-bacterial filters.
There are also adverse effects to water treatment and purification devices which arise from the presence of microbes and their decomposition products in water, and which are problematic to human health. For example, water purification devices often experience what is referred to microbial “grow-through”. Media filters, sand filters, carbon filters, clay filters, and others have limited life spans due to the growth of bacteria. As a result, these materials become septic and malodorous and release cellular decomposition products as part of their effluent. These decomposition products, in turn, foul and interfere with the efficient operation of downstream treatment technologies such as membrane filtration and ion exchange systems. Therefore, the presence of cellular decomposition products in water can cause human health concerns through various routes of entry. Although poisons such as silver and bactericides may kill microbes, they do nothing to address the microbial decomposition products, and their use can in some cases actually enhance the pathogenicity of the water.
Therefore, what are needed are non-toxic compositions and methods that provide antimicrobial activity, which can address the need for safer and more environmentally benign ways to protect various substrates, media, and surfaces from microbial growth and proliferation. One possible approach would be to develop ways that could help prevent the EPS from fulfilling its role in cellular organization and cohesion. It would be desirable if the non-toxic compositions and methods could not simply exhibit antimicrobial activity, but also address the problems from endotoxins that may arise from that antimicrobial activity. Desirably, these materials would not result in negative effects on flora, fauna, and humans.